Method for producing water expandable styrene polymers

ABSTRACT

The invention relates to a process for preparing styrene polymers which comprise water as sole blowing agent, by polymerizing styrene in aqueous suspension in the presence of from 0.1 to 15% by weight of a solid substance insoluble in water and in styrene, for example carbon black or graphite.

The invention relates to a process for preparing water-expandablepolystyrene (WEPS) by polymerizing styrene in aqueous suspension, wherethe suspended styrene droplets comprise an emulsion of finely dispersedwater.

Expandable polystyrene (EPS) beads are usually prepared by polymerizingstyrene in aqueous suspension in the presence of a volatile organicblowing agent. The usual blowing agents are hydrocarbons, in particularpentane. In order to protect the environment, pentane emitted during theproduction and processing of EPS has to be reclaimed. This iscomplicated and costly. In the longer term, therefore, it would beuseful to replace these organic substances with blowing agents which areless hazardous, for example water.

The Eindhoven University 1997 dissertation by J. J. Crevecoeur “WaterExpandable Polystyrene” describes a process for preparing WEPS by firstpreparing a fine emulsion of water in styrene with the aid ofsurface-active substances, polymerizing the styrene to a conversion of50%, suspending the mixture in water with phase inversion, and finallypolymerizing the styrene to completion with the aid of peroxideinitiators. The surface-active substances used comprise amphiphilicemulsifiers, e.g. sodium [bis(2-ethylhexyl)sulfosuccinate] or sodiumstyrenesulfonate, or block copolymers made of polystyrene blocks and ofpolystyrenesulfonate blocks. All of these substances have both ahydrophilic and a hydrophobic radical and are therefore able to emulsifywater in styrene.

A disadvantage of this process is that it is carried out in two stages:water is first emulsified in the styrene/polystyrene mixture and thenthe organic phase is suspended in water, with phase inversion.

It is an object of the present invention, therefore, to develop asimpler, single-stage process for preparing WEPS.

We have found that this object is achieved by adding, at the start ofthe suspension polymerization or during its course, from 0.1 to 15% byweight, based on the monomers, of an inorganic solid substance insolublein water and in styrene and having an average particle size of not morethan 100 μm and a density above 1.1 g/cm³.

During the polymerization, gravity and centrifugal forces dictate thatthe particles of the solid substance continuously penetrate theinterface between water phase and styrene phase and while doing thisconstantly carry adhering water droplets with them into the styrenephase. This is probably the basis for the action of the solids asemulsifying agents.

Preferred solid substance is carbon black with an average particle sizeof from 10 to 500 nm, depending on the type of carbon black. Even thepreferred amounts of from 0.2 to 5% by weight of carbon black areeffective. Another suitable substance is graphite with an averageparticle size (longest lamellar diameter) of from 2 to 20 μm. Activeamounts are preferably from 0.4 to 10% by weight.

Other suitable substances are silica gel, silicates, e.g. talc orbentonite, metal oxides, e.g. Al₂O₃ and TiO₂, hydroxides, such asAlO(OH) and Mg(OH)₂, and also metal salts, such as CaCO₃, Mg₃(PO₄) andBaSO₄.

In the novel suspension polymerization it is preferable for styrenealone to be used as monomer. However, up to 20% of the weight of styrenemay have been replaced by other ethylenically unsaturated monomers, suchas alkylstyrenes, divinylbenzene, acrylonitrile, 1,1-diphenylethene orα-methylstyrene.

In the suspension polymerization use may be made of the customaryauxiliaries, e.g. suspension stabilizers, free-radical initiators, flameretardants, chain transfer agents, expansion aids, nucleating agents andplasticizers. It is advantageous for the suspension stabilizers used tobe inorganic Pickering dispersing agents, e.g. magnesium pyrophosphateor tricalcium phosphate, combined with small amounts of alkylsulfonates.Preferred flame retardants are organic bromine compounds, such ashexabromocyclododecane, and the amounts of these added are from 0.1 to2% by weight, based on the monomers.

It is advantageous to carry out the polymerization in the presence offrom 1 to 30% by weight, preferably from 3 to 15% by weight, ofpolystyrene, and it is appropriate for this to be used as a solution instyrene. It is also possible to use recycled polystyrene materialinstead of pure polystyrene.

The solid substance is preferably added straight away at the start ofthe suspension polymerization, but it may also be fed during the courseof the polymerization until the conversion is 90%.

It is appropriate for the suspension polymerization to be carried out intwo temperature phases, using two peroxide initiators decomposing atdifferent temperatures. The suspension is first heated to 80-90° C.,whereupon the first peroxide, e.g. dibenzoyl peroxide, decomposes andthe polymerization begins. The temperature is then allowed slowly torise to 100-140° C., whereupon the second peroxide, e.g. dicumylperoxide or di-tert-butyl perbenzoate, decomposes.

The WEPS beads produced during the suspension polymerization comprise,depending on the amount of recycled EPS material used and the content ofcoating agent, from 2 to 20% by weight, in particular from 5 to 15% byweight, of water. Their particle size is from 0.2 to 5 mm, preferablyfrom 0.5 to 2 mm. They may be foamed using air at from 110 to 140° C. orsuperheated steam, to give foam beads. A particularly elegant foamingprocess which gives foam beads with a very low bulk density is describedin the German Patent Application P 198 12 854.1.

The WEPS foam beads may, like conventional EPS foam beads, be fused togive foam sheets, foam slabs or foam moldings, each of which can be usedas an insulating or packaging material.

All percentages in the examples are based on weight.

EXAMPLE 1

2.55 kg of polystyrene (PS 158 K from BASF) are dissolved in 17.03 kg ofstyrene, and 510 g (3%) of pulverulent graphite (Graphitwerk KropfmUhleKG, UF2 96/97, average particle size 4.5 μm) are homogeneously suspendedwith admixture of 59.6 g of dicumyl peroxide and 20.4 g of dibenzoylperoxide. The organic phase is introduced into 19.5 l of demineralizedwater in a 50 l stirred vessel. The aqueous phase comprises 69.8 g ofsodium pyrophosphate and 129.5 g of magnesium sulfate. The suspension isheated, with stirring, to 80° C. and 3.51 g of sodiumsec.-C₁₅-alkylsulfonate are added, whereupon the suspension stabilizersystem forms. The stirrer rotation rate here was 140 rpm. Finally, thepolymerization is completed at 134° C. Removal of the aqueous phasegives bead-shaped pellets comprising 8% of water.

Using air heated above 100° C., the product could be expanded to one 8thof its initial bulk density of about 600 g/l. The prefoamed product wasthen dried and foamed using steam in a second and a third expansion stepto a bulk density of 10 g/l.

EXAMPLE 2

2.55 kg of polystyrene (PS 158 K from BASF) are dissolved in 17.03 kg ofstyrene, and 340 g (2%) of pulverulent carbon black (MT N 990, DegussaAG, average particle size 320 nm) are homogeneously suspended withadmixture of 59.6 g of dicumyl peroxide and 20.4 g of dibenzoylperoxide. The organic phase is introduced into 19.5 l of demineralizedwater in a 50 l stirred vessel. The aqueous phase comprises 69.8 g ofsodium pyrophosphate and 129.5 g of magnesium sulfate. The suspension isheated, with stirring, to 800° C. The stirrer rotation rate here was 140rpm. After 140 minutes 3.51 g of alkyl sulfonate are added. Finally, thepolymerization is completed at 134° C. Removal of the aqueous phasegives bead-shaped pellets comprising 11% of water.

Using air heated above 100° C., the product could be expanded to one10th of its initial bulk density of about 600 g/l. The prefoamed productwas then dried and foamed using steam in a second and a third expansionstep to a bulk density of less than 10 g/l.

We claim:
 1. A process for preparing expandable polystyrene beads whichcomprises water as sole blowing agent by polymerizing styrene in aqueoussuspension in the presence of magnesium pyrophosphate or tricalciumphosphate as suspension stabilizers, where the suspended styrenedroplets comprise an emulsion of finely dispersed water and anemulsifying agent is present, which comprises adding, as emulsifyingagent, at the start of the suspension polymerization or during itscourse, from 0.1 to 15% by weight, based on styrene, of carbon black orgraphite having an average particle size of not more than 100 μm and adensity above 1.1 g/cm³.
 2. A process as claimed in claim 1, whereincarbon black with an average particle size of from 10 to 500 nm orgraphite with an average particle size (longest lamellar diameter) offrom 2 to 20 μm is added.
 3. A process as claimed in claim 1, whereinthe solid substance is added to the polymerization batch at a conversionof from 0 to 90%.
 4. A process as claimed in claim 1, wherein thepolymerization is carried out in the presence of from 1 to 30% byweight, based on styrene, of polystyrene dissolved in the monomer.
 5. Amethod for producing foamed polystyrene beads, comprising: foamingexpandable polystyrene beads comprising 2 to 20% by weight of water andprepared as set forth in claim 1 with air at a temperature ranging from110 to 140° C. or by superheated steam.
 6. A process for preparingexpandable polystyrene beads which comprises water as sole blowing agentby polymerizing styrene in aqueous suspension in the presence ofmagnesium pyrophosphate or tricalcium phosphate as suspensionstabilizers, where the suspended styrene droplets comprise an emulsionof finely dispersed water and an emulsifying agent is present, whichcomprises adding, as emulsifying agent, at the start of the suspensionpolymerization or during its course, from 0.1 to 15% by weight, based onstyrene, of carbon black or graphite having an average particle size ofnot more than 100 μm and a density above 1.1 g/cm³, the expandablepolystyrene beads containing from 2 to 20% by weight water.